RFID tag location using synthesized-beam RFID readers

ABSTRACT

Synthesized-beam RFID readers may be used to locate RFID tags. In one embodiment, a tag&#39;s response rates on different beams can be used, along with the target locations of those beams, to estimate the tag&#39;s location. The estimated tag location is within a region where beams with nonzero tag response rates overlap, and the distances of the estimated tag location from any two different beam target locations may correspond to a ratio of tag response rates on the two different beams. In another embodiment, a tag&#39;s response rates on different beam pairs configured to cooperatively power RFID tags can be used, along with the target locations of those beam pairs, to estimate the tag&#39;s location.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part under 35 U.S.C. § 120 of aco-pending U.S. patent application Ser. No. 15/075,214 filed on Mar. 21,2016, which is a continuation of U.S. Pat. No. 9,373,012 issued on Jun.21, 2016, which was a national phase application of InternationalApplication no. PCT/US14/26319 filed on Mar. 13, 2014 which claimspriority to U.S. Provisional Patent Application Ser. No. 61/784,035filed on Mar. 14, 2013 and 61/887,238 filed on Oct. 4, 2013. Thedisclosures of these provisional patent applications are herebyincorporated by reference for all purposes.

BACKGROUND

Radio-Frequency Identification (RFID) systems typically include RFIDreaders, also known as RFID reader/writers or RFID interrogators, andRFID tags. RFID systems can be used in many ways for locating andidentifying objects to which the tags are attached. RFID systems areuseful in product-related and service-related industries for trackingobjects being processed, inventoried, or handled. In such cases, an RFIDtag is usually attached to an individual item, or to its package.

In principle, RFID techniques entail using an RFID reader to inventoryone or more RFID tags, where inventorying involves at least singulatinga tag and receiving an identifier from the singulated tag. “Singulated”is defined as a reader singling-out one tag, potentially from amongmultiple tags, for a reader-tag dialog. “Identifier” is defined as anumber identifying the tag or the item to which the tag is attached,such as a tag identifier (TID), electronic product code (EPC), etc. Thereader transmitting a Radio-Frequency (RF) wave performs theinterrogation. The RF wave is typically electromagnetic, at least in thefar field. The RF wave can also be predominantly electric or magnetic inthe near or transitional near field. The RF wave may encode one or morecommands that instruct the tags to perform one or more actions.

In typical RFID systems, an RFID reader transmits a modulated RFinventory signal (a command), receives a tag reply, and transmits an RFacknowledgement signal responsive to the tag reply. A tag that sensesthe interrogating RF wave may respond by transmitting back another RFwave. The tag either generates the transmitted back RF wave originally,or by reflecting back a portion of the interrogating RF wave in aprocess known as backscatter. Backscatter may take place in a number ofways.

The reflected-back RF wave may encode data stored in the tag, such as anumber. The response is demodulated and decoded by the reader, whichthereby identities, counts, or otherwise interacts with the associateditem. The decoded data can denote a serial number, a price, a date, atime, a destination, an encrypted message, an electronic signature,other attribute(s), any combination of attributes, and so on.Accordingly, when a reader receives tag data it can learn about the itemthat hosts the tag and/or about the tag itself.

An RFID tag typically includes an antenna section, a radio section, apower-management section, and frequently a logical section, a memory, orboth. In some RFID tags the power-management section included an energystorage device such as a battery. RFID tags with an energy storagedevice are known as battery-assisted, semi-active, or active tags. OtherRFID tags can be powered solely by the RF signal they receive. Such RFIDtags do not include an energy storage device and are called passivetags. Of course, even passive tags typically include temporary energy-and data/flag-storage elements such as capacitors or inductors.

BRIEF SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended asan aid in determining the scope of the claimed subject matter.

Embodiments are directed to locating RFID tags using synthesized-beamRFID readers. A synthesized-beam RFID reader, which comprises at leastone RFID reader and an antenna array, electrically synthesizes multiplebeam patterns by adjusting the signals provided to the antenna elementsof the array. The multiple beam patterns may point in different physicaldirections, may provide different beam shapes, may provide differentphysical coverage, or a may provide mix of these attributes. The readermay comprise single or multiple transmitters, single or multiplereceivers, be separate from and connected to elements of the antennaarray, or be distributed and embedded within the elements of the array.Either the reader or an array controller may adjust the phase and/oramplitude of the signals provided to the array elements to synthesizethe multiple beams. The antenna array may comprise multiple discreteantenna elements or may employ a continuous structure that can emulatemultiple antennas. By switching among the beams, a synthesized-beamreader may scan its environment, essentially steering its gaze indifferent directions and with potentially different beam shapes as itscans. As a simple but not-limiting example of a synthesized-beamsystem, consider the antenna array on a U.S. Navy ship that forms asynthesized-beam radar, and envision the radar scanning the environmentto inventory RFID tags rather than scanning the environment to detectdistant ships or airplanes. Like a synthesized-beam radar, asynthesized-beam RFID reader may use multiple RF frequencies, differentbeam shapes, different beam directions, and different signal waveshapesto inventory/locate/track its target tags.

Synthesized-beam RFID readers may be used to locate RFID tags. In oneembodiment, a tag's response rates on different beams can be used, alongwith the target locations of those beams, to estimate the tag'slocation. The estimated tag location is within a region where beams withnonzero tag response rates overlap, and the distances of the estimatedtag location from any two different beam target locations may correspondto a ratio of tag response rates on the two different beams. In anotherembodiment, a tag's response rates on different beam pairs configured tocooperatively power RFID tags can be used, along with the targetlocations of those beam pairs, to estimate the tag's location.

According to some examples, a method to estimate a location of a RadioFrequency Identification (RFID) integrated circuit (IC) coupled to anantenna is provided. The method includes generating multipleradio-frequency beams, where each beam is directed to a different targetlocation, transmitting multiple interrogating signals on each beam, andreceiving, on each beam, at least one response from the IC to theinterrogating signals. The method may further include determining aresponse rate for each beam, selecting a first beam having a firstresponse rate and a second beam having a second response rate, where thefirst beam partially overlaps the second beam to form an overlap region,and using a target location of the first beam, a target location of thesecond beam, and the first and second response rates to estimate the IClocation within the overlap region.

According to other examples, a method to estimate a location of an RFIDIC coupled to an antenna is provided. The method includes generatingmultiple pairs of radio-frequency (RF) beams, where each beam pair isdirected to a different target location and the beams within each beampair cooperatively provide RF power to the target location, transmittingmultiple interrogating signals on at least one beam of each beam pair,and receiving, on at least one beam of each beam pair, at least oneresponse from the IC to the interrogating signals. The method mayfurther include determining a response rate for each beam pair,selecting a first beam pair having a first response rate and a secondbeam pair having a second response rate, where the first beam pairpartially overlaps the second beam pair to form an overlap region, andusing a target location of the first beam pair, a target location of thesecond beam pair, and the first and second response rates to estimatethe IC location within the overlap region.

According to further examples, a method for an RFID synthesized-beamreader to estimate a location of an RFID IC coupled to an antenna isprovided. The method includes serially synthesizing each of multiplebeams according to a scan pattern, where each beam is directed to adifferent target location, transmitting a series of interrogatingsignals on each beam, and receiving, on each beam, at least one responsefrom the IC to the series of interrogating signals. The method mayfurther include determining a response rate for each beam, selecting afirst beam having a first response rate and a second beam having asecond response rate, where the first beam partially overlaps the secondbeam to form an overlap region, and using a target location of the firstbeam, a target location of the second beam, and the first and secondresponse rates to estimate the IC location within the overlap region.

These and other features and advantages will be apparent from a readingof the following detailed description and a review of the associateddrawings. It is to be understood that both the foregoing generaldescription and the following detailed description are explanatory onlyand are not restrictive of aspects as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The following Detailed Description proceeds with reference to theaccompanying drawings, in which:

FIG. 1 is a block diagram of components of an RFID system.

FIG. 2 is a diagram showing components of a passive RFID tag, such as atag that can be used in the system of FIG. 1.

FIG. 3 is a conceptual diagram for explaining a half-duplex mode ofcommunication between the components of the RFID system of FIG. 1.

FIG. 4 is a block diagram showing a detail of an RFID IC.

FIGS. 5A and 5B illustrate signal paths during tag-to-reader andreader-to-tag communications in the block diagram of FIG. 4.

FIG. 6 is a block diagram of a whole RFID reader system according toembodiments.

FIG. 7 is a block diagram illustrating an overall architecture of anRFID system according to embodiments.

FIG. 8 depicts a discrete-element antenna array according toembodiments.

FIGS. 9A and 9B depict the antenna array of FIG. 8 synthesizing a beamin different physical directions, according to embodiments.

FIG. 10 depicts some of the potential beam locations that can besynthesized by the antenna array of FIG. 8, according to embodiments.

FIG. 11 depicts radiated beam power as a function of beam angle for asubset of the potential beams of the antenna array of FIG. 8.

FIG. 12 depicts an RFID tag located in the subset of the potential beamsof FIG. 11.

FIG. 13 depicts how the location of the RFID tag of FIG. 12 can bedetermined using tag response rates.

FIG. 14 depicts how a tag's location may be determined using multiplebeams.

FIG. 15 depicts beams with non-circular beam shapes formed by asynthesized-beam reader according to embodiments.

FIG. 16 depicts beams with sidelobes formed by a synthesized-beam readeraccording to embodiments.

FIG. 17 depicts how frequency-based variations in beam power can be usedto determine tag location.

FIG. 18 depicts a process for determining tag location by counting tagreads of a synthesized-beam reader.

FIG. 19 depicts the effective tag inventory range of a synthesized-beamreader according to embodiments.

FIG. 20 depicts how a synthesized-beam reader's effective tag inventoryrange can be increased using another synthesized-beam reader accordingto embodiments.

FIG. 21 depicts how multiple synthesized-beam readers can cooperate tocommunicate with a population of tags according to embodiments.

FIG. 22 depicts methods of controlling multiple synthesized-beam readersaccording to embodiments.

FIG. 23 depicts a process for using cooperating synthesized-beam readersto enhance tag inventory range according to embodiments.

FIG. 24 is a conceptual diagram showing side views of a synthesized-beamreader at different stages of a tag motion-tracking process according toembodiments.

FIG. 25 depicts a timing diagram for a tag tracking process with tagrefresh commands according to embodiments.

FIG. 26 is a flowchart of a tag tracking process according toembodiments.

DETAILED DESCRIPTION

In the following detailed description, references are made to theaccompanying drawings that form a part hereof, and in which are shown byway of illustration specific embodiments or examples. These embodimentsor examples may be combined, other aspects may be utilized, andstructural changes may be made without departing from the spirit orscope of the present disclosure. The following detailed description istherefore not to be taken in a limiting sense, and the scope of thepresent invention is defined by the appended claims and theirequivalents.

As used herein, “memory” is one of ROM. RAM, SRAM. DRAM, NVM, EEPROM,FLASH, Fuse, MRAM, FRAM, and other similar information-storagetechnologies as will be known to those skilled in the art, and may bevolatile or not. Some portions of memory may be writeable and some not.“Command” refers to a reader request for one or more tags to perform oneor more actions, and includes one or more tag instructions preceded by acommand identifier or command code that identifies the command and/orthe tag instructions. “instruction” refers to a request to a tag toperform a single explicit action (e.g., write data into memory).“Program” refers to a request to a tag to perform a set or sequence ofinstructions (e.g., read a value from memory and, if the read value isless than a threshold then lock a memory word). “Protocol” refers to anindustry standard for communications between a reader and a tag (andvice versa), such as the Class-1 Generation-2 UHF RFID Protocol forCommunications at 860 MHz-960 MHz by GS1 EPCglobal, Inc. (“Gen2Specification”), versions 1.2.0 and 2.0 of which are hereby incorporatedby reference.

FIG. 1 is a diagram of the components of a typical RFID system 100,incorporating embodiments. An RFID reader 110 transmits an interrogatingRF signal 112. RFID tag 120 in the vicinity of RFID reader 110 sensesinterrogating RF signal 112 and generate signal 126 in response. RFIDreader 110 senses and interprets signal 126. The signals 112 and 126 mayinclude RF waves and/or non-propagating RF signals (e.g., reactivenear-field signals).

Reader 110 and tag 120 communicate via signals 112 and 126. Whencommunicating, each encodes, modulates, and transmits data to the other,and each receives, demodulates, and decodes data from the other. Thedata can be modulated onto, and demodulated from, RF waveforms. The RFwaveforms are typically in a suitable range of frequencies, such asthose near 900 MHz, 13.56 MHz, and so on.

The communication between reader and tag uses symbols, also called RFIDsymbols. A symbol can be a delimiter, a calibration value, and so on.Symbols can be implemented for exchanging binary data, such as “0” and“1”, if that is desired. When symbols are processed by reader 110 andtag 120 they can be treated as values, numbers, and so on.

Tag 120 can be a passive tag, or an active or battery-assisted tag(i.e., a tag having its own power source). When tag 120 is a passivetag, it is powered from signal 112.

FIG. 2 is a diagram of an RFID tag 220, which may function as tag 120 ofFIG. 1. Tag 220 is drawn as a passive tag, meaning it does not have itsown power source. Much of what is described in this document, however,applies also to active and battery-assisted tags.

Tag 220 is typically (although not necessarily) formed on asubstantially planar inlay 222, which can be made in many ways known inthe art. Tag 220 includes a circuit which may be implemented as an IC224. In some embodiments IC 224 is implemented in complementarymetal-oxide semiconductor (CMOS) technology. In other embodiments IC 224may be implemented in other technologies such as bipolar junctiontransistor (BJT) technology, metal-semiconductor field-effect transistor(MESFET) technology, and others as will be well known to those skilledin the art. IC 224 is arranged on inlay 222.

Tag 220 also includes an antenna for exchanging wireless signals withits environment. The antenna is often flat and attached to inlay 222. IC224 is electrically coupled to the antenna via suitable IC contacts (notshown in FIG. 2). The term “electrically coupled” as used herein meansthat a low-impedance path exists between the electrically coupledcomponents, and may mean the presence of a direct electrical connectionor a connection that includes one or more intervening circuit blocks,elements, or devices. The “electrical” part of the term “electricallycoupled” as used in this document shall mean a coupling that is one ormore of ohmic/galvanic, capacitive, and/or inductive. Similarly, theterm “electrically isolated” as used herein means that electricalcoupling of one or more types (e.g., galvanic, capacitive, and/orinductive) is not present, at least to the extent possible. For example,elements that are electrically isolated from each other are galvanicallyisolated from each other, capacitively isolated from each other, and/orinductively isolated from each other. Of course, electrically isolatedcomponents will generally have some unavoidable stray capacitive orinductive coupling between them, but the intent of the isolation is tominimize this stray coupling to a negligible level when compared with anelectrically coupled path.

IC 224 is shown with a single antenna port, comprising two IC contactselectrically coupled to two antenna segments 226 and 228 which are shownhere forming a dipole. Many other embodiments are possible using anynumber of ports, contacts, antennas, and/or antenna segments.

Diagram 250 depicts top and side views of tag 252, formed using a strap.Tag 252 differs from tag 220 in that it includes a substantially planarstrap substrate 254 having strap contacts 256 and 258. IC 224 is mountedon strap substrate 254 such that the IC contacts on IC 224 electricallycouple to strap contacts 256 and 258 via suitable connections (notshown). Strap substrate 254 is then placed on inlay 222 such that strapcontacts 256 and 258 electrically couple to antenna segments 226 and228. Strap substrate 254 may be affixed to inlay 222 via pressing, aninterface layer, one or more adhesives, or any other suitable means.

Diagram 260 depicts a side view of an alternative way to place strapsubstrate 254 onto inlay 222. Instead of strap substrate 254's surface,including strap contacts 256/258, facing the surface of inlay 222, strapsubstrate 254 is placed with its strap contacts 256/258 facing away fromthe surface of inlay 222. Strap contacts 256/258 can then be eithercapacitively coupled to antenna segments 226/228 through strap substrate254, or conductively coupled using a through-via which may be formed bycrimping strap contacts 256/258 to antenna segments 226/228. In someembodiments the positions of strap substrate 254 and inlay 222 may bereversed, with strap substrate 254 mounted beneath inlay 222 and strapcontacts 256/258 electrically coupled to antenna segments 226/228through inlay 222. Of course, in yet other embodiments strap contacts256/258 may electrically couple to antenna segments 226/228 through bothinlay 222 and strap substrate 254.

In operation, the antenna receives a signal and communicates it to IC224, which may both harvest power and respond if appropriate, based onthe incoming signal and the IC's internal state. If IC 224 usesbackscatter modulation then it responds by modulating the antenna'sreflectance, which generates response signal 126 from signal 112transmitted by the reader. Electrically coupling and uncoupling the ICcontacts of IC 224 can modulate the antenna's reflectance, as canvarying the admittance of a shunt-connected circuit element which iscoupled to the IC contacts. Varying the impedance of a series-connectedcircuit element is another means of modulating the antenna'sreflectance. If IC 224 is capable of transmitting signals (e.g., has itsown power source, is coupled to an external power source, and/or is ableto harvest sufficient power to transmit signals), then IC 224 mayrespond by transmitting response signal 126.

In the embodiments of FIG. 2, antenna segments 226 and 228 are separatefrom IC 224. In other embodiments the antenna segments may alternativelybe formed on IC 224. Tag antennas according to embodiments may bedesigned in any form and are not limited to dipoles. For example, thetag antenna may be a patch, a slot, a loop, a coil, a horn, a spiral, amonopole, microstrip, stripline, or any other suitable antenna.

An RFID tag such as tag 220 is often attached to or associated with anindividual item or the item packaging. An RFID tag may be fabricated andthen attached to the item or packaging, or may be partly fabricatedbefore attachment to the item or packaging and then completelyfabricated upon attachment to the item or packaging. In someembodiments, the manufacturing process of the item or packaging mayinclude the fabrication of an RFID tag. In these embodiments, theresulting RFID tag may be integrated into the item or packaging, andportions of the item or packaging may serve as tag components. Forexample, conductive item or packaging portions may serve as tag antennasegments or contacts. Nonconductive item or packaging portions may serveas tag substrates or inlays. If the item or packaging includesintegrated circuits or other circuitry, some portion of the circuitrymay be configured to operate as part or all of an RFID tag IC.

The components of the RFID system of FIG. 1 may communicate with eachother in any number of modes. One such mode is called full duplex, whereboth reader 110 and tag 120 can transmit at the same time. In someembodiments. RFID system 100 may be capable of full duplex communicationif tag 120 is configured to transmit signals as described above. Anothersuch mode, suitable for passive tags, is called half-duplex, and isdescribed below.

FIG. 3 is a conceptual diagram 300 for explaining half-duplexcommunications between the components of the RFID system of FIG. 1, inthis case with lag 120 implemented as passive tag 220 of FIG. 2. Theexplanation is made with reference to a TIME axis, and also to a humanmetaphor of “talking” and “listening”. The actual technicalimplementations for “talking” and “listening” are now described.

RFID reader 110 and RFID tag 120 talk and listen to each other by takingturns. As seen on axis TIME, when reader 110 talks to tag 120 thecommunication session is designated as “R→T”, and when tag 120 talks toreader 110 the communication session is designated as “T→R”. Along theTIME axis, a sample R→T communication session occurs during a timeinterval 312, and a following sample T→R communication session occursduring a time interval 326. Interval 312 may typically be of a differentduration than interval 326—here the durations are shown approximatelyequal only for purposes of illustration.

According to blocks 332 and 336, RFID reader 110 talks during interval312, and listens during interval 326. According to blocks 342 and 346,RFID tag 120 listens while reader 110 talks (during interval 312), andtalks while reader 110 listens (during interval 326).

In terms of actual behavior, during interval 312 reader 110 talks to tag120 as follows. According to block 352, reader 110 transmits signal 112,which was first described in FIG. 1. At the same time, according toblock 362, tag 120 receives signal 112 and processes it to extract dataand so on. Meanwhile, according to block 372, tag 120 does notbackscatter with its antenna, and according to block 382, reader 110 hasno signal to receive from tag 120.

During interval 326, tag 120 talks to reader 110 as follows. Accordingto block 356, reader 110 transmits a Continuous Wave (CW) signal, whichcan be thought of as a carrier that is typically not amplitude modulatedor phase modulated and therefore encodes no information. This CW signalserves both to transfer energy to tag 120 for its own internal powerneeds, and also as a carrier that tag 120 can modulate with itsbackscatter. Indeed, during interval 326, according to block 366, tag120 does not receive a signal for processing. Instead, according toblock 376, tag 120 modulates the CW emitted according to block 356 so asto generate backscatter signal 126. Concurrently, according to block386, reader 110 receives backscatter signal 126 and processes it.

FIG. 4 is a block diagram showing a detail of an RFID IC, such as IC 224in FIG. 2. Electrical circuit 424 in FIG. 4 may be formed in an IC of anRFID tag, such as tag 220 of FIG. 2. Circuit 424 has a number of maincomponents that are described in this document. Circuit 424 may have anumber of additional components from what is shown and described, ordifferent components, depending on the exact implementation.

Circuit 424 shows two IC contacts 432, 433, suitable for coupling toantenna segments such as antenna segments 226/228 of RFID tag 220 ofFIG. 2. When two IC contacts form the signal input from and signalreturn to an antenna they are often referred-to as an antenna port. ICcontacts 432, 433 may be made in any suitable way, such as from metallicpads and so on. In some embodiments circuit 424 uses more than two ICcontacts, especially when tag 220 has more than one antenna port and/ormore than one antenna.

Circuit 424 includes signal-routing section 435 which may include signalwiring, signal-routing busses, receive/transmit switches, and so on thatcan route a signal to the components of circuit 424. In some embodimentsIC contacts 432/433 couple galvanically and/or inductively tosignal-routing section 435. In other embodiments (such as is shown inFIG. 4) circuit 424 includes optional capacitors 436 and/or 438 which,if present, capacitively couple IC contacts 432/433 to signal-routingsection 435. This capacitive coupling causes IC contacts 432/433 to begalvanically decoupled from signal-routing section 435 and other circuitcomponents.

Capacitive coupling (and resultant galvanic decoupling) between ICcontacts 432 and/or 433 and components of circuit 424 is desirable incertain situations. For example, in some RFID tag embodiments ICcontacts 432 and 433 may galvanically connect to terminals of a tuningloop on the tag. In this situation, capacitors 436 and/or 438galvanically decouple IC contact 432 from IC contact 433, therebypreventing the formation of a short circuit between the IC contactsthrough the tuning loop.

Capacitors 436/438 may be implemented within circuit 424 and/or partlyor completely external to circuit 424. For example, a dielectric orinsulating layer on the surface of the IC containing circuit 424 mayserve as the dielectric in capacitor 436 and/or capacitor 438. Asanother example, a dielectric or insulating layer on the surface of atag substrate (e.g., inlay 222 or strap substrate 254) may serve as thedielectric in capacitors 436/438. Metallic or conductive layerspositioned on both sides of the dielectric layer (i.e., between thedielectric layer and the IC and between the dielectric layer and the tagsubstrate) may then serve as terminals of the capacitors 436/438. Theconductive layers may include IC contacts (e.g., IC contacts 432/433),antenna segments (e.g., antenna segments 226/228), or any other suitableconductive layers.

Circuit 424 also includes a rectifier and PMU (Power Management Unit)441 that harvests energy from the RF signal received by antenna segments226/228 to power the circuits of IC 424 during either or bothreader-to-tag (R→T) and tag-to-reader (T→R) sessions. Rectifier and PMU441 may be implemented in any way known in the art.

Circuit 424 additionally includes a demodulator 442 that demodulates theRF signal received via IC contacts 432, 433. Demodulator 442 may beimplemented in any way known in the art, for example including a slicer,an amplifier, and so on.

Circuit 424 further includes a processing block 444 that receives theoutput from demodulator 442 and performs operations such as commanddecoding, memory interfacing, and so on. In addition, processing block444 may generate an output signal for transmission. Processing block 444may be implemented in any way known in the art, for example bycombinations of one or more of a processor, memory, decoder, encoder,and so on.

Circuit 424 additionally includes a modulator 446 that modulates anoutput signal generated by processing block 444. The modulated signal istransmitted by driving IC contacts 432, 433, and therefore driving theload presented by the coupled antenna segment or segments. Modulator 446may be implemented in any way known in the art, for example including aswitch, driver, amplifier, and so on.

In one embodiment, demodulator 442 and modulator 446 may be combined ina single transceiver circuit. In another embodiment modulator 446 maymodulate a signal using backscatter. In another embodiment modulator 446may include an active transmitter. In yet other embodiments demodulator442 and modulator 446 may be part of processing block 444.

Circuit 424 additionally includes a memory 450 to store data 452. Atleast a portion of memory 450 is preferably implemented as a NonvolatileMemory (NVM), which means that data 452 is retained even when circuit424 does not have power, as is frequently the case for a passive RFIDtag.

In some embodiments, particularly in those with more than one antennaport, circuit 424 may contain multiple demodulators, rectifiers, PMUs,modulators, processing blocks, and/or memories.

In terms of processing a signal, circuit 424 operates differently duringa R→T session and a T→R session. The different operations are describedbelow, in this case with circuit 424 representing an IC of an RFID tag.

FIG. 5A shows version 524-A of components of circuit 424 of FIG. 4,further modified to emphasize a signal operation during a R→T sessionduring time interval 312 of FIG. 3. Demodulator 442 demodulates an RFsignal received from IC contacts 432, 433. The demodulated signal isprovided to processing block 444 as C_IN. In one embodiment, C_IN mayinclude a received stream of symbols.

Version 524-A shows as relatively obscured those components that do notplay a part in processing a signal during a R→T session. Rectifier andPMU 441 may be active, such as for converting RF power. Modulator 446generally does not transmit during a R→T session, and typically does notinteract with the received RF signal significantly, either becauseswitching action in section 435 of FIG. 4 decouples modulator 446 fromthe RF signal, or by designing modulator 446 to have a suitableimpedance, and so on.

Although modulator 446 is typically inactive during a R→T session, itneed not be so. For example, during a R→T session modulator 446 could beadjusting its own parameters for operation in a future session, and soon.

FIG. 5B shows version 524-B of components of circuit 424 of FIG. 4,further modified to emphasize a signal operation during a T→R sessionduring time interval 326 of FIG. 3. Processing block 444 outputs asignal C_OUT. In one embodiment, C_OUT may include a stream of symbolsfor transmission. Modulator 446 then modulates C_OUT and provides it toantenna segments such as segments 226/228 of RFID tag 220 via ICcontacts 432, 433.

Version 524-B shows as relatively obscured those components that do notplay a part in processing a signal during a T→R session. Rectifier andPMU 441 may be active, such as for converting RF power. Demodulator 442generally does not receive during a T→R session, and typically does notinteract with the transmitted RF signal significantly, either becauseswitching action in section 435 of FIG. 4 decouples demodulator 442 fromthe RF signal, or by designing demodulator 442 to have a suitableimpedance, and so on.

Although demodulator 442 is typically inactive during a T→R session, itneed not be so. For example, during a T→R session demodulator 442 couldbe adjusting its own parameters for operation in a future session, andso on.

In typical embodiments, demodulator 442 and modulator 446 are operableto demodulate and modulate signals according to a protocol, such as theGen2 Specification mentioned above. In embodiments where circuit 424includes multiple demodulators and/or modulators, each may be configuredto support different protocols or different sets of protocols. Aprotocol specifies, in part, symbol encodings, and may include a set ofmodulations, rates, timings, or any other parameter associated with datacommunications. In addition, a protocol can be a variant of a statedspecification such as the Gen2 Specification, for example includingfewer or additional commands than the stated specification calls for,and so on. In such instances, additional commands are sometimes calledcustom commands.

FIG. 6 is a block diagram of an RFID reader system 600 according toembodiments. RFID reader system 600 includes a local block 610, andoptionally remote components 670. Local block 610 and remote components670 can be implemented in any number of ways. For example, local block610 or portions of local block 610 may be implemented as a standalonedevice or as a component in another device. In some embodiments, localblock 610 or portions of local block 610 may be implemented as a mobiledevice, such as a handheld RFID reader, or as a component in a mobiledevice, such as a laptop, tablet, smartphone, wearable device, or anyother suitable mobile device. It will be recognized that RFID reader 110of FIG. 1 is the same as local block 610, if remote components 670 arenot provided. Alternately, RFID reader 110 can be implemented instead byRFID reader system 600, of which only the local block 610 is shown inFIG. 1.

In some embodiments, one or more of the blocks or components of readersystem 600 may be implemented as integrated circuits. For example, localblock 610, one or more of the components of local block 610, and/or oneor more of the remote component 670 may be implemented as integratedcircuits using CMOS technology, BJT technology, MESFET technology,and/or any other suitable implementation technology.

Local block 610 is responsible for communicating with RFID tags. Localblock 610 includes a block 651 of an antenna and a driver of the antennafor communicating with the tags. Some readers, like that shown in localblock 610, contain a single antenna and driver. Some readers containmultiple antennas and drivers and a method to switch signals among them,including sometimes using different antennas for transmitting and forreceiving. Some readers contain multiple antennas and drivers that canoperate simultaneously. In some embodiments, block 651 may be aphased-array antenna or synthesized-beam antenna (SBA), described inmore detail below, and local block 610 may be implemented in asynthesized-beam reader (SBR) configured to generate one or more beamsvia the SBA. A demodulator/decoder block 653 demodulates and decodesbackscattered waves received from the tags via antenna/driver block 651.Modulator/encoder block 654 encodes and modulates an RF wave that is tobe transmitted to the tags via antenna/driver block 651.

Local block 610 additionally includes an optional local processor 656.Local processor 656 may be implemented in any number of ways known inthe art. Such ways include, by way of examples and not of limitation,digital and/or analog processors such as microprocessors anddigital-signal processors (DSPs); controllers such as microcontrollers;software running in a machine such as a general purpose computer,programmable circuits such as Field Programmable Gate Arrays (FPGAs),Field-Programmable Analog Arrays (FPAAs), Programmable Logic Devices(PLDs), Application Specific Integrated Circuits (ASIC), any combinationof one or more of these; and so on. In some cases, some or all of thedecoding function in block 653, the encoding function in block 654, orboth, may be performed instead by local processor 656. In some caseslocal processor 656 may implement an encryption or authenticationfunction; in some cases one or more of these functions can bedistributed among other blocks such as encoding block 654, or may beentirely incorporated in another block.

Local block 610 additionally includes an optional local memory 657.Local memory 657 may be implemented in any number of ways known in theart, including, by way of example and not of limitation, any of thememory types described above as well as any combination thereof. Localmemory 657 can be implemented separately from local processor 656, or inan IC with local processor 656, with or without other components. Localmemory 657, if provided, can store programs for local processor 656 torun, if needed.

In some embodiments, local memory 657 stores data read from tags, ordata to be written to tags, such as Electronic Product Codes (EPCs), TagIdentifiers (TIDs) and other data. Local memory 657 can also includereference data that is to be compared to EPCs, instructions and/or rulesfor how to encode commands for the tags, modes for controlling antenna651, encryption/authentication algorithms, algorithms for tracking taglocation or movement, secret keys, key pairs, individual public and/orprivate keys, electronic signatures, and so on. In some of theseembodiments, local memory 657 is provided as a database.

Some components of local block 610 typically treat the data as analog,such as the antenna/driver block 651. Other components such as localmemory 657 typically treat the data as digital. At some point there is aconversion between analog and digital. Based on where this conversionoccurs, a reader may be characterized as “analog” or “digital”, but mostreaders contain a mix of analog and digital functionality.

If remote components 670 are provided, they are coupled to local block610 via an electronic communications network 680. Network 680 can be aLocal Area Network (LAN), a Metropolitan Area Network (MAN), a Wide AreaNetwork (WAN), a network of networks such as the internet, or a localcommunication link, such as a USB, PCI, and so on. Local block 610 mayinclude a local network connection 659 for communicating withcommunications network 680 or may couple to a separate device orcomponent configured to communicate with communications network 680.Communications on the network can be secure, such as if they areencrypted or physically protected, or insecure if they are not encryptedor otherwise protected.

There can be one or more remote component(s) 670. If more than one, theycan be located at the same location, or in different locations. They maycommunicate with each other and local block 610 via communicationsnetwork 680, or via other similar networks, and so on. Accordingly,remote component(s) 670 can use respective remote network connections.Only one such remote network connection 679 is shown, which is similarto local network connection 659, etc.

Remote component(s) 670 can also include a remote processor 676. Remoteprocessor 676 can be made in any way known in the art, such as wasdescribed with reference to local processor 656. Remote processor 676may also implement an encryption/authentication function and/or a taglocation/tracking function, similar to local processor 656.

Remote component(s) 670 can also include a remote memory 677. Remotememory 677 can be made in any way known in the art, such as wasdescribed with reference to local memory 657. Remote memory 677 mayinclude a local database, and a different database of a standardsorganization, such as one that can reference EPCs. Remote memory 677 mayalso contain information associated with commands, tag profiles, keys,or the like, similar to local memory 657.

One or more of the above-described elements may be combined anddesignated as operational processing block 690. Operational processingblock 690 includes those components that are provided of the following:local processor 656, remote processor 676, local network connection 659,remote network connection 679, and by extension an applicable portion ofcommunications network 680 that links remote network connection 679 withlocal network connection 659. The portion can be dynamically changeable,etc. In addition, operational processing block 690 can receive anddecode RF waves received via antenna/driver 651, and causeantenna/driver 651 to transmit RF waves according to what it hasprocessed.

Operational processing block 690 includes either local processor 656, orremote processor 676, or both. If both are provided, remote processor676 can be made such that it operates in a way complementary with thatof local processor 656. In fact, the two can cooperate. It will beappreciated that operational processing block 690, as defined this way,is in communication with both local memory 657 and remote memory 677, ifboth are present.

Accordingly, operational processing block 690 is location independent,in that its functions can be implemented either by local processor 656,or by remote processor 676, or by a combination of both. Some of thesefunctions are preferably implemented by local processor 656, and some byremote processor 676. Operational processing block 690 accesses localmemory 657, or remote memory 677, or both for storing and/or retrievingdata.

RFID reader system 600 operates by operational processing block 690generating communications for RFID tags. These communications areultimately transmitted by antenna/driver block 651, withmodulator/encoder block 654 encoding and modulating the information onan RF wave. Then data is received from the tags via antenna/driver block651, demodulated and decoded by demodulator/decoder block 653, andprocessed by operational processing block 690.

Embodiments of an RFID reader system can be implemented as hardware,software, firmware, or any combination. Such a system may be subdividedinto components or modules. Some of these components or modules can beimplemented as hardware, some as software, some as firmware, and some asa combination. An example of such a subdivision is now described,together with the RFID tag as an additional module.

FIG. 7 is a block diagram illustrating an overall architecture of anRFID system 700 according to embodiments. RFID system 700 may besubdivided into modules or components, each of which may be implementedby itself or in combination with others. In addition, some of them maybe present more than once. Other embodiments may be equivalentlysubdivided into different modules. Some aspects of FIG. 7 are parallelwith systems, modules, and components described previously.

An RFID tag 703 is considered here as a module by itself. RFID tag 703conducts a wireless communication 706 with the remainder, via the airinterface 705. Air interface 705 is really a boundary, in that signalsor data that pass through it are not intended to be transformed from onething to another. Specifications as to how readers and tags are tocommunicate with each other, for example the Gen2 Specification, alsoproperly characterize that boundary as an interface.

RFID system 700 includes one or more reader antennas 710, and an RFfront-end module 720 for interfacing with reader antenna(s) 710. Thesecan be made as described above.

RFID system 700 also includes a signal-processing module 730. In oneembodiment, signal-processing module 730 exchanges waveforms with RFfront-end module 720, such as I and Q waveform pairs.

RFID system 700 further includes a physical-driver module 740, which isalso known as a data-link module. In some embodiments physical-drivermodule 740 exchanges bits with signal-processing module 730.Physical-driver module 740 can be the stage associated with the framingof data.

RFID system 700 additionally includes a media access control module 750.In one embodiment, media access control layer module 750 exchangespackets of bits with physical driver module 740. Media access controllayer module 750 can make decisions for sharing the medium of wirelesscommunication, which in this case is the air interface.

RFID system 700 moreover includes an application-programminglibrary-module 760. This module 760 can include application programminginterfaces (APIs), other objects, etc.

All of these RFID system functionalities can be supported by one or moreprocessors. One of these processors can be considered a host processor.Such a host processor might include a host operating system (OS) and/orcentral processing unit (CPU), as in module 770. In some embodiments,the processor is not considered as a separate module, but one thatincludes some of the above-mentioned modules of RFID system 700. In someembodiments the one or more processors may perform operations associatedwith retrieving data that may include a tag public key, an electronicsignature, a tag identifier, an item identifier, and/or asigning-authority public key. In some embodiments the one or moreprocessors may verify an electronic signature, create a tag challenge,and/or verify a tag response.

User interface module 780 may be coupled toapplication-programming-library module 760, for accessing the APIs. Userinterface module 780 can be manual, automatic, or both. It can besupported by the host OS/CPU module 770 mentioned above, or by aseparate processor, etc.

It will be observed that the modules of RFID system 700 form a chain.Adjacent modules in the chain can be coupled by appropriateinstrumentalities for exchanging signals. These instrumentalitiesinclude conductors, buses, interfaces, and so on. Theseinstrumentalities can be local, e.g. to connect modules that arephysically close to each other, or over a network, for remotecommunication.

The chain is used in one direction for receiving RFID waveforms and inthe other direction for transmitting RFID waveforms. In receiving mode,reader antenna(s) 710 receives wireless waves, which are in turnprocessed successively by the various modules in the chain. Processingcan terminate in any one of the modules. In transmitting mode, waveforminitiation can be in any one of the modules. Ultimately, signals arerouted to reader antenna(s) 710 to be transmitted as wireless waves.

The architecture of RFID system 700 is presented for purposes ofexplanation, and not of limitation. Its particular, subdivision intomodules need not be followed for creating embodiments. Furthermore, thefeatures of the present disclosure can be performed either within asingle one of the modules, or by a combination of them. In someembodiments RFID system 700 can be incorporated into another electronicdevice such as a checkout terminal in a store or a consumer device suchas a mobile phone.

One or more RFID readers, or distributed portions of one or morereaders, may be coupled to or embedded within an antenna array to form asynthesized-beam reader (SBR) capable of generating multiple RF beams,as described above. FIG. 8 depicts a perspective view of an antennaarray 800 with discrete radiating elements according to embodiments.Antenna array 800 includes an array of antenna elements 802 and 804, anda ground plane 808 behind elements 802 and 804. Each element has aradiating direction vector 806 (only shown for one element) that istypically, but not necessarily, perpendicular to the ground plane. An RFradiation pattern (or “beam”) for receiving or transmitting an RF signalmay be synthesized by adjusting the amplitude and/or phase of thesignals coupled from/to each antenna element 802 and 804. Theorientation or direction of the synthesized beam (typically representedby the direction of the beam's primary lobe—the lobe having the highestradiated power) is controlled by these various amplitude and/or phaseadjustments. The adjustments may be analog, digital, or a mix of analogand digital. For example, during transmission, an SBR may generate thesignal to be transmitted and then direct the generated signal toelements 802 and 804 with different amplitudes and phases.Alternatively, the SBR may synthesize the different signals for eachantenna element digitally and then convert the digital signals toanalog. For example, each antenna element can be implemented as aseparate digital transceiver having its own analog front end. Controlsignals to generate a beam can then be supplied to the different digitaltransceivers, each of which converts a digital signal into an analogsignal for transmission. When the digital transceivers transmit theiranalog signals, the signals combine to form the synthesized beam. Inother embodiments the SBR may use a mix of these approaches. Similarly,during a receive operation the SBR may combine analog signals afterappropriate phase shifting and amplitude adjustment of each, or it maydigitize the signals from each element and combine them digitally, or amix thereof.

The antenna elements of SBA 800 may be one or more of patch, slot, wire,horn, helical, distributed, or any other type as will be known to thoseskilled in the art. Whereas FIG. 8 only shows nine antenna elements,antenna arrays with any number of antenna elements may be used,including a single distributed element or an element made frommetamaterials. In some embodiments ground plane 808 may be nonplanar(e.g., curved, concave, convex, etc.) and in other embodiments need notexist.

FIGS. 9A and 9B show the orientations of some of the RF beams that SBA900, similar to SBA 800 in FIG. 8, can generate. SBA 900 has nineantenna elements 902-918, with element 902 at the center and elements904-918 around it. The shape and direction of the beam that SBA 900generates depends on the signals to/from each element. Suppose that SBR900 transmits using primarily elements 902, 906, and 914. Then,depending on the amplitude and phase of the signals applied to theseelements, SBA 900 can change the orientation of a beam (also referred toas “steering” the beam) along the direction indicated by dashed line920. In a similar fashion, suppose that SBR 900 transmits primarilyusing elements 902, 908, and 916. Then, depending on the amplitude andphase of the signals applied to these elements, SBA 900 can steer a beamalong the direction indicated by dashed line 922. Of course, othersteering arrangements are possible, including using all 9 elements totransmit and/or receive in arbitrary directions and to generate narrowbeams.

FIG. 9B shows how RF beams with different directions can be synthesizedusing antenna elements located along line 920, with the diagram to theleft depicting a head-on view similar to FIG. 9A and the diagram to theright depicting a side view. As described above, the beam direction canbe controlled by varying the amplitude and phase of the signals to/fromthe antenna elements. For example, by applying a leading signal phase toelement 906, an intermediate signal phase to element 902, and a trailingsignal phase to element 914, the SBA will tend to steer its beamdownward as in beam 934. Switching leading and lagging from elements906/902 to elements 902/906 will tend to steer the beam upwards as inbeam 930. Of course, the actual beam shape depends on both the magnitudeof the phase shifting and the magnitude of the amplitude scaling (ifany).

FIG. 10 depicts potential beams from an SBR according to embodiments.Diagram 1000 depicts a side perspective of SBR 1010, capable ofsynthesizing at least five different RF beams 1012, 1013, 1014, 1015,and 1016, arranged along line 1018 (similar to line 920 in FIG. 9A),with each RF beam pointed in a different direction.

A beam can be characterized by one or more longitudinal beamcross-sections as depicted in diagram 1000 (that is, cross-sections ofthe beam in one or more planes parallel to the beam direction) and/orperpendicular beam cross-sections (that is, cross-sections of the beamin one or more planes perpendicular to the beam direction). A beam canalso be characterized by a beam length indicative of the powerdistribution of the beam along the beam direction, a beam widthindicative of the power distribution of the beam in a directionperpendicular to the beam direction, and/or any other suitableshape-based parameter. Beam shapes may be based on, for example, thetype of antenna used, the RF frequency of the beam, the power used togenerate the beam, and/or how the beam is transmitted. Insynthesized-beam embodiments, the beam illumination may be based on thearrangement of excited antenna elements and the amplitude, phase, and/orfrequency of the various signals used to excite the antenna elements.

Diagrams 1020, 1040, 1060, and 1080 depict example coverage areas, shownas shaded circles, of the beam patterns generated by SBR 1010, A beamgenerated by an SBR has a coverage volume, also known as the beam's“field-of-view (FoV)”, which is a volume in three-dimensional spacewhere, during transmission, the transmitted energy density exceeds athreshold, and where, during receiving, the received energy densityexceeds a threshold. A beam's coverage area is a projection of thebeam's FoV on a surface. The FoV and coverage area may be differentduring transmit and receive, and may vary with reader or tag power, thethresholds, the distance between the SBR and the surface, and otherparameters. For example, a beam may have different FoVs and thereforecoverage areas based on the threshold(s) selected for transmitted and/orreceived energy densities.

Diagram 1020 depicts an example coverage area of central beam 1012.Diagram 1040 depicts example coverage areas of the inner beams such as1014 and 1015. Diagram 1060 depicts example coverage areas of the outerbeams such as 1015 and 1016. Finally, diagram 1080 depicts an exampletotal coverage area of all the beams formed by SBR 1010. As shown indiagrams 1020-1080, beam coverage areas may overlap. For example, innerbeam 1014 may overlap with the central beam 1012, with one or more otherinner beams, and with one or more other outer beams.

Whereas SBR 1010 is depicted as being able to generate and switchbetween five beams on an axis (e.g., axis 1018), in other embodiments anSBR may generate and switch between more or fewer beams on any givenaxis. Similarly, whereas SBR 1010 is depicted as being able to generatebeams on four different axes (e.g., axes 920, 922, 924, and 926 in FIG.9A), in other embodiments an SBR may be configured to generate beams onmore or fewer axes. An individual beam's coverage area in FIG. 10 andsubsequent figures is depicted as circular for simplicity, and inactuality may be of any suitable shape, and may vary based oninteractions between the different elements that form the beam, as wellas the orientation and topology of the surface on which the coveragearea is projected. For example, a beam may have a non-circular coveragearea. As another example, a circular beam that illuminates a surfacewith a non-perpendicular angle may project an elliptical coverage areaon the surface.

FIG. 11 depicts receive sensitivity or beam power as a function of beamangle for a subset of the potential beams of the SBR of FIG. 10.Sensitivity indicates the ease with which a receiver (reader or tag) canrecover an incident waveform, and can be measured in terms of power.Diagram 1100 is similar to diagram 1000, with similarly numberedelements behaving similarly. Diagram 1120 depicts a view of theoverlapping areas of coverage of beams 1012, 1013, 1014, 1015, and 1016,all lying along axis 1018. Chart 1140 plots receive sensitivity or beampower as a function of beam angle. Each of the beams 1012-1016 isoriented at a particular angle, with beam 1012 oriented perpendicular tothe plane of SBR 1010, beams 1013/1014 oriented at an angle θ from beam1012, and beams 1015/1016 oriented at an angle φ from beam 1012. Theseangles are mapped on the horizontal axis 1142 of chart 1140. Thevertical axis 1144 may represent either beam sensitivity or deliveredpower (or both), with sensitivity/power decreasing away from the origin.Each of the beams 1012-1016 has a sensitivity/power contour depicted asan arc in chart 1140 with a maximum at the beam's pointing angle. Thecontours define three-dimensional surfaces for which the receivesensitivity, beam power, or both, have a constant value. If we considerjust beam power, as the tag's position moves away from the beam'spointing angle, the power decreases, reducing the beam's ability topower the tag. Alternatively, the power contour can represent theminimum power needed to power the RFID tag, and the curves in the beam'scontour can represent the reduction in read range as the tag moves awayfrom the beam's pointing angle. Of course, beam sensitivity/power mayvary with beam RF frequency, tag sensitivity, desired tag operation, ormyriad other RFID parameters as will be known to those skilled in theart.

FIG. 12 is a diagram depicting an RFID tag located such that it isilluminated by a subset of the beams of FIG. 11. Chart 1200 plots beampower 1202 available to power the tag from beam 1012. Horizontal axis1206 represents position and vertical axis 1204 power. A tag's abilityto extract power from an incident RF wave varies with RF frequency, formany reasons, including the fact that a tag antenna is typically moresensitive at certain frequencies than others. Line 1210 represents thepower an incident RF wave at the worst-case (i.e. least sensitive) RFfrequency must have in order for the tag to operate, and line 1222represents the power an incident RF wave at the best-case (i.e. mostsensitive) RF frequency must have in order for the tag to operate. Thetag is able to operate at all physical locations and RF frequencies forwhich curve 1202 lies above line 1210, and is able to operate at allphysical locations and the best-case RF frequency for which curve 1202lies above line 1218. In other words, the intersection points of curve1202 and line 1210, when extended to a multi-dimensional space, bound abeam region in which the tag is able to operate at all RF frequencies,and the intersection points of curve 1202 and line 1218, when extendedto a multi-dimensional space, bound a beam region in which the tag isable to operate at the best-case RF frequency. Of course, there exist acontinuum of lines between 1210 and 1218, representing the continuum ofRF frequencies between best and worst case, and a correspondingcontinuum of bound beam regions representing different regions in whichthe tag can operate. This continuum of frequencies is represented by tagoperating profile 1219.

Diagrams 1220 and 1240 are similar to diagrams 1120 and 1140, withsimilarly numbered elements behaving similarly. Diagram 1240 is similarto diagram 1200, but with tag position converted from physical positionto beam pointing angle. Tag 1222, shown with an “X” in diagram 1220 and1240, is located where multiple beams overlap, and can be powered by abeam when the beam's power contour lies above line 1218 for the tag'sbest-case operating frequency. Line 1210 is not shown in diagram 1240for reasons on clarity, but tag operating profile 1219 shows the rangeof power levels at various frequencies for which the tag is able tooperate. It is clear that some beams can power the tag at allfrequencies; some beams at some frequencies; and some beams at nofrequencies. Moreover, it is clear that a tag's location with respect toa beam's area-of-coverage affects whether the tag can be powered by thebeam. For example, tag 1222 lies near the center of beam 1012'sarea-of-coverage, and accordingly beam 1012 can power tag 1222 over allRF frequencies. In contrast, tag 1222 is relatively far from the centerof beam 1015's area-of-coverage, and accordingly beam 1015 can onlypower tag 1222 over a few RF frequencies, if any at all.

The relationship between whether a beam can power a tag and the distancebetween the beam's center (alternatively, the beam's target location)and the tag can be used to estimate the tag's location. FIG. 13 depictshow the location of the RFID tag 1222 of FIG. 12 can be determined usingtag response rates. FIG. 13 includes a series of charts (1310, 1320,1330, and 1340) and area-of-coverage diagrams (1312, 1322, 1332, and1342) of the subset of potential beams of FIG. 12. Tag 1222 isrepresented as an “X” in the area-of-coverage diagrams, and tagoperating profile 1219 shows the range of power levels at various RFfrequencies for which the tag is able to operate.

Consider diagrams 1310 and 1312, which depict beam 1012 interacting withtag 1222. Tag 1222 is relatively centered within beam 1012's area ofcoverage. As a result, the entire tag operating profile 1219 lies withinthe contour of beam 1012, and the tag is able to operate regardless ofbeam 1012's RF frequency. As a result, when the SBR transmitsinterrogating signals on beam 1012 on multiple RF frequencies, the SBRwill receive responses from tag 1222 at each frequency, resulting in a100% response rate with frequency, depicted as “[100]” in diagram 1312.A tag's response rate is representative of the number of responsesreceived from a tag over some other parameter. For example, a tag'sresponse rate may be a ratio of the count of responses from the tagreceived on a beam to the number of interrogating signals transmitted onthe beam, or may be a ratio of the tag's response count to a period oftime over which interrogating signals were transmitted. In someembodiments, tag response rates may be based on inventory cycles, asdescribed in commonly-assigned U.S. patent application Ser. No.15/201,231 filed on Jul. 1, 2016, hereby incorporated by reference inits entirety.

Turning now to beam 1013, notice that tag 1222 is farther from thecenter of beam 1013 than from the center of beam 1012. As a result, thetag operating profile 1219 exceeds beam 1013's power for a small subsetof frequencies, whereas the tag operating profile 1219 did not exceedbeam 1012's power for any frequencies. Accordingly, when the SBRtransmits interrogating signals on beam 1013 on multiple RF frequencies,the SBR will receive responses from tag 1222 at only a subset offrequencies. This result in a 92% response rate with frequency for beam1013, depicted as “[92]” in diagram 1322, which is lower than theresponse rate with frequency for beam 1012.

Turning to beam 1014, notice that tag 1222 is even farther from thecenter of beam 1014 than the centers of beams 1012 and 1013, which meansthat tag operating profile 1219 exceeds beam 1013's power for an evenlarger subset of frequencies. As a result, when the SBR transmitsinterrogating signals on beam 1014 on multiple frequencies, the SBR willreceive responses from tag 1222 over an even smaller subset offrequencies, resulting in a 60% response rate with frequency, depictedas “[60]” in diagram 1332, lower than the response rates for beams 1012and 1013.

Finally, diagrams 1340 and 1342 depict how each of the five beams1012-1016 may interact with tag 1222. As described above, an SIRswitching between beams 1012, 1013, and 1014 and varying RF frequencieson those beams may have response rates of 100%, 92%, and 60% wheninteracting with tag 1222. For outer beams 1015 and 1016, whose centersare far from the tag 1222 and whose contours barely overlap the tagrange marker if at all, the response rates are 2% (“[2]”) and 0%(“[0]”).

Because responses from tag 1222 are received when interrogating signalsare transmitted on each of beams 1012-1015, the location of tag 1222 canbe estimated to be within an overlap region, where beams 1012-1015 allpartially overlap. The location of tag 1222 within the overlap regioncan be further refined based on measured beam response rates. In thisspecific example, beam 1012 has a 100% response rate for tag 1222, sotag 1222 lies near the center of its coverage area. The response ratesfor tag 1222 on beams 1013 and 1014 are smaller than the response ratefor beam 1012, which means that tag 1222 is farther from the centers ofbeams 1013 and 1014 than from the center of beam 1012. Moreover, beam1013 has a larger response rate for tag 1222 than beam 1014, so tag 1222is closer to the center of beam 1013 than to the center of beam 1014.

In some embodiments, the ratios between different beam response ratescan be used to estimate tag location. In diagram 1342, beam 1012 has a100% response rate for tag 1222, whereas beam 1013 has a 92% responserate for tag 1222. Accordingly, the location of tag 1222 may beestimated such that a ratio of a distance between the location and beam1012's target location (or beam center) to a distance between thelocation and beam 1013's target location corresponds to the ratiobetween the beam 1012's response rate and beam 1013's response rate. Thelocation estimation can be further refined by using ratios associatedwith other sets of beams. For example, the location of tag 1222 can befurther refined such that a ratio of a distance between the location andbeam 1013's target location to a distance between the location and beam1014's target location corresponds to the ratio between the beam 1013'sresponse rate and beam 1014's response rate, and so on.

Different ratios may correspond to each other in different ways. Forexample, two ratios may correspond to each other if they have the samevalue or substantially the same value. Two ratios may also correspond toeach other if, when at least one of the ratios is scaled by arespective, not-necessarily-identical scaling factor, both ratios havesubstantially the same value. In some embodiments, two ratios maycorrespond to each other if, when at least one ratio is used as inputinto a respective, not-necessarily-identical algorithm, the algorithmoutput(s) have substantially the same value as each other or the otherratio.

In some embodiments, weighted averaging or centroid-averaging methodscan be used to scale response rates for tag location estimation. Inthese embodiments, tag response rates are scaled or weighted such thatbeams with higher tag response rates are more heavily weighted in thelocation estimation than beams with lower tag response rates. Tagresponse rates may be weighted by exponentiation, by multiplication witha static or dynamic factor, or by any suitable method. In someembodiments, the slopes of the beam sensitivity responses can becompared to the tag operating profile to refine tag location.

Whereas the tag location in FIG. 12 was determined, for reasons ofbrevity and clarity, using beams lying along axis 1018, other beamsalong other axes may also be used. FIG. 14 depicts how a tag's locationmay be determined using multiple beams in 2-dimensional space. Diagram1400 depicts beam patterns similar to those in diagram 1080 of FIG. 10.For clarity, each beam in diagram 1400 is represented by a small shadedcircle that does not necessarily represent its coverage area. Tag 1422,similar to tag 1222, is represented by an “X”. Each beam has anassociated tag response rate (as described in FIG. 13) represented bythe bracketed number in the beam's circle. The response rate for a beamis related to the distance between the center of the beam's coveragearea and the location of tag 1422. Beams whose centers are closer to tag1422 have higher response rates than beams whose centers are fartherfrom tag 1422. The location of tag 1422 can then be determined using thedifferent response rates of the different beams, for example using theratio-based and/or averaging approaches described above.

Although FIGS. 13 and 14 show specific response rates, these values areprovided for illustration, and may not correspond to actual responserates. Similarly, the relationships between the response rates providedby adjacent beams shown in FIGS. 13 and 14 are for illustration, and maydiffer from the actual response rates between adjacent beams.

Whereas a beam's coverage area is depicted as circular in the figuresabove, in some embodiments a beam's coverage area may be shaped. As oneexample, FIG. 15 depicts beams with elliptical shapes formed by an SBRaccording to embodiments.

Diagram 1500 in FIG. 15 is similar to FIG. 9B and depicts beamssynthesized by activating antenna elements along plane 920. As describedin FIG. 9B, RF beams 1530, 1532, and 1534 (similar to RF beams 930, 932,and 934, respectively) can be synthesized by supplyingappropriately-phase-shifted signals to the antenna elements along plane920. Because only 3 antenna elements are activated, in a line, the SBRof diagram 1500 is able to steer and shape a beam in the verticaldirection in diagram 1500 but not in the horizontal direction. Ofcourse, activating additional elements can shift and shape the beam inother directions. Diagram 1540 shows elements activated along line 922to generate a beam of shape 1542, and elements activated along line 924to generate a beam of shape 1546. Adjusting the phase of the signalsprovided to the antenna elements along lines 922, 924, and 926 allowssteering beams to different locations, as shown in diagram 1560 forbeams 1562, 1564, and 1566, respectively. As described above, a tag atlocation 1546 in diagram 1560 may produce different response rates withfrequency to an SBR steering a beam in these various directions,allowing the SBR to determine the tag's location.

All steered-beam systems generate side beams (sidelobes) in addition tothe main beam. As will be known to those skilled in the art, there existtechniques to suppress these sidelobes, such as by adding more steeringelements, digital beam shaping, shielding, and other methods, but nonecan suppress the sidelobes entirely.

FIG. 16 depicts steered beams with sidelobes formed by an SBR accordingto embodiments. Diagram 1600, similar to FIG. 9B, depicts a head-on viewto the left and a side view to the right of an SBR generating an RF beam1630 having a sidelobe 1632, which may inventory a tag that is notpositioned along the beam's main lobe. For example, beam 1630 pointsupward, but sidelobe 1632, pointing downward, can erroneously inventorytag 1634. Fortunately, because sidelobes have lower power than mainlobes, an SBR can typically use a tag's response rate to discriminate atag in a main beam (which will have a high response rate) from a tag ina sidelobe (which will have a low response rate). If a detected tag isdetermined to be located in a sidelobe of the beam, the response ratefor that tag in that beam may be excluded during estimation of taglocation. In some embodiments, the sidelobe response rate may beadjusted via some algorithm and subsequently used to refine tag locationestimation.

As described above, a tag's sensitivity and response rate may vary withthe frequency of the incident beam. A beam's sensitivity contour alsovaries with frequency, and these variations may be used to estimate taglocation. FIG. 17 depicts how frequency-based variations in beam powercan be used to determine tag location. Graph 1700 shows a beam havingsensitivity contour 1702 associated with a first radio frequency andsensitivity contour 1704 associated with a second radio frequency. Graph1700 also depicts a tag operating profile 1719 similar to profile 1219.By using beam profiles along with the tag response rates as a functionof frequency, an SBR may be able to better estimate the tag's location.In one embodiment, the ratio-based tag location estimation processdescribed above may use a frequency-based response rate comparison,where, for each radio frequency, response rates at different beams arecompared. The comparison can be performed for each different radiofrequency, and tag location estimation can use the comparisons at thedifferent radio frequencies. For example, a tag's location may beestimated such that a ratio of distances between the estimated locationand the target locations of two different beams correspond to both (a) aratio of response rates of the two beams at a first radio frequency and(b) a ratio of response rates of the two beams at a second radiofrequency.

FIG. 18 depicts a process 1800 for determining tag location by countingtag reads. Process 1800 begins at operation 1802, where a tag isinventoried by an SBR, on one or more of its generated beams. Atoperation 1804, the SBR sweeps through different beams and differentradio frequencies for each beam, counting the tag response rate inoperation 1806. For example, the SBR may generate and count tag readsusing a first beam at a first radio frequency, then generate and counttag reads using a second beam at the first radio frequency, and continuethis process until it has iterated through all beams. The SBR may thenrepeat the process for different radio frequencies. Alternatively, theSBR may count tag reads using a first beam at a first radio frequency,then generate and count tag reads using the first beam at a second radiofrequency, and continue this process until it has iterated through allradio frequencies. The SBR may then repeat the process for differentbeams. Of course, the SBR may interleave these choices of beams andradio frequencies in any way imaginable, including randomly. The SBR mayact adaptively/intelligently, choosing a subset of beams, and radiofrequencies that allow it to locate a tag as quickly and/or accuratelyas possible. The SBR may alter the polarization of its beams to reducethe impact of tag polarization when determining tag location. The SBRmay combine beam/frequency data with other information, such as a map orplanogram of a facility (e.g. the SBR may exclude known barriers such aswalls from the potential locations for a tag) or tag type (e.g. sometags are more sensitive or directional than others. Finally, atoperation 1808, the SBR uses the collected beam, radio frequency,response rate, and other information to determine the tag location. Forexample, the SBR can estimate the distance of the tag from the centersor target locations of at least two beams by comparing tag response datafor the beams, where a higher tag response for a beam indicates that thetag is closer to that beam. The SBR can perform the distance estimationfor multiple sets of beams, then use the estimated distances to identifyan intersecting region or a region where multiple beams overlap withinwhich the tag is located.

The tag localization techniques described above may be modified in anysuitable way. For example, tag location may be performed with two ormore separate readers, stationary readers, mobile readers, or anycombination of the previous. In some embodiments the tag localizationtechniques described herein may be used to determine the instantaneouslocation and direction-of-travel of moving tags, and may also be used todetermine whether a particular tag is stationary or in motion, forexample as described in more detail below. The localization or locationprocess may be further refined based on some other technique orparameter, such as interpolation, averaging, knowledge of the physicalconfiguration associated with the tag location and/or zone, historicaldetection data associated with the tag, other data associated with thetag (e.g., purchase data, type of tag, type of item associated with thetag, etc.), data associated with received tag responses (e.g., receivedsignal strength indicator or RSSI, angle-of-arrival, phase, a Dopplerparameter or shift of the response, response timing, data included inthe response), or any other suitable technique or parameter.

When tracking tags associated with a stationary or moving structure, areader or location system may adjust tag response rates based on one ormore other parameters associated with the structures. For example, tagresponse rates may be adjusted based on structure movement speed anddirection, structure physical configuration, structure orientation,and/or structure composition. A location system may be able to determinethe movement speed and direction of a structure based on previousdetections of tags associated with the structure, or may receive thespeed and direction information from an external source, such as aconveyor system or transporter associated with the structure. Tagresponse rates may be affected by structure physical configuration dueto RF blocking or interference from structural components interposedbetween readers and tags. Similarly, tag response rates may be affectedby structure composition. A metallic structure may block, attenuate,and/or interfere with RF signals between readers and tags than a plasticstructure.

A beam's ability to inventory or access a tag is based on the absolutebeam power, beam power contour, beam pointing direction, beam frequency,distance to the tag, beam and tag polarization, type of tag, type of tagoperation (e.g. inventory versus writing), reader sensitivity, tagsensitivity, and other parameters. When a tag is within a reader's taginventory range, the tag has sufficient power to receive and respond toreader commands and the reader has sufficient sensitivity tosuccessfully receive tag responses. When a tag lies outside a reader'stag inventory range, either the tag may not have sufficient power toreceive and respond to reader commands, the reader may not havesufficient sensitivity to successfully receive tag responses, or both.

FIG. 19 depicts an SBR's tag inventory range according to embodiments.Diagrams 1920 and 1940 show side and head-on views of an SBR 1900attempting to inventory tags 1904 and 1906 using a beam 1902. Each ofthe tags 1904/1906 has an associated sensitivity contour, labeled as1908 and 1910, respectively. SBR 1900 can inventory tag 1904 using beam1902, because beam 1902's power contour overlaps tag sensitivity contour1908. SBR 1900 cannot inventory tag 1906 using beam 1902, because beam1902 does not overlap tag 1906's sensitivity contour 1910.

Suppose that there was a way to deliver additional power to tag 1906 andthereby increase the size of its sensitivity contour 1910. Asufficiently large increase would allow beam 1902 to inventory tag 1906.Of course, one way to increase the size of tag 1906's sensitivitycontour is to add a battery to tag 1906. However, it may be desirable tofind a way to do so without artificial means such as a battery.

FIG. 20 depicts how a tag's sensitivity contour can be increased byusing a cooperating SBR to deliver additional RF power, according toembodiments. FIG. 20, similar to FIG. 19, depicts side and head-on views2020 and 2040, respectively, of SBR 1900 attempting to inventory tags1904 and 1906. FIG. 20 also includes another SBR 2000 positioned todeliver additional RF power to tag 1910 using beam 2002. In FIG. 20 asdepicted, neither SBR 1900 nor SBR 2000 can inventory tag 1906 on itsown, because beams 1902 and 2002 lie outside sensitivity contour 1910 oftag 1906. However, if SBR 1900 and SBR 2000 deliver RF power at the sametime, then the effective sensitivity contour of tag 1906 increases fromperimeter 1910 to perimeter 2010, allowing both SBR's to inventory tag1906. In practice, there are many ways for SBR 1900 and SBR 2000 tocooperate. SBR 1900 can issue inventory commands while SBR 2000 sendsraw RF power (a continuous or minimally modulated wave). Or SBR 1900 cansend raw RF power while SBR 2000 issues inventory commands. Or both SBR1900 and SBR 2000 can issue synchronized inventory commands. Regardlessof the means, two SBRs can deliver more power than one, and if the twoSBRs are synchronized to cooperatively provide power than tag 1906,which was invisible to either SBR singly, can become visible to one orboth of them.

In some embodiments, the two SBRs may use the same RF frequency duringsuch cooperative powering, particularly in cases where the SBRs sendsynchronized commands. In other embodiments, particularly those in whichone SBR provides commands and the other raw RF power, the SBRs may usesignificantly different frequencies to avoid the RF frequenciesgenerating beat notes that confuse the tag's demodulator (such asdemodulator 442 in FIG. 4). Of course, the SBRs may optimize theirchoice of frequency based on beam or tag polarization, tag type, thetag's ability to reject interference such as beat notes, and otherparameters. The SBR's may also adjust their relative or absolutedelivered power to improve the cooperative powering. In some embodimentsone of the SBRs need not be an SBR at all, but could instead be a fixed(i.e. not steered beam) reader, a handheld reader, a shelf reader, adock-door reader, or any other RFID reader as will be known to thoseskilled in the art. In some embodiments a controller and/or one or bothof the SBRs can adjust the power and/or frequency of the generated RFsignal(s) to increase the inventorying range of the RFID tag. In someembodiments the adjustment may include sweeping over a range of powerand/or frequency values; in other embodiments the adjustment may beadaptive and based on environmental conditions (for example, RF noise orthe dielectric properties of items to which tags are attached), receivedtag replies, tag performance characteristics (for example, sensitivity,interference rejection or ability to harvest power from another RFsource), and/or tag population size. In some embodiments, one of theSBRs will point to the target location and another SBR may steer itsbeam to multiple locations in the vicinity of the target location toimprove the cooperative powering.

Of course, the benefits of cooperative tag powering need not be limitedto a pair of SBRs. FIG. 21 depicts how multiple SBRs can cooperate toinventory a tag population according to embodiments. Diagram 2100depicts four SBRs 2102, 2104, 2106, and 2108 arranged to inventory a tagpopulation 2110. In diagram 2100. SBR 2102 is configured to communicatewith tags 2110 using beam 2112, while SBRs 2104-2108 are configured toprovide power to tags 2110 using beams 2114, 2116, and 2118,respectively. In diagram 2150, SBR 2108 is instead configured toinventory tags 2110 using beam 2118, while SBRs 2102-2106 provideadditional power. Of course, any of the SBRs can do the inventoryingwhile the others deliver power, or two can inventory (synchronously ornot) while the other two deliver power, or any other possibility. Insome embodiments, one or more of the SBRs may be configured to receivesignals from tags 2110 without generating beams, while the other SBRsgenerate beams to transmit inventory commands and/or provide power totags 2110, similar to the operation of a bistatic radar system. Forexample, SBR 2102 may be configured to receive signals from tags 2110without generating beam 2112, while SBRs 2104-2108 generate beams2114-2118, respectively, to transmit inventory commands and/or providepower to tags 2110. In some embodiments, SBRs configured to receivesignals from tags 2110 may generate beams with relatively lower powerthan other SBRs configured to primarily transmit inventory commandsand/or provide power to tags 2110, potentially increasing the receivesensitivity of the receiving SBRs. Of course, the number of SBRs isarbitrary—four are shown but more or less can be used in actualpractice.

In some embodiments a controller, such as controller 2120 in diagrams2100 and 2150, can perform SBR coordination to cooperatively inventoryand power tags using SBRs 2102, 2104, 2106, and 2108. In otherembodiments controller 2120 can be embedded within one or more of theSBRs. In yet other embodiments the SBRs can form a peer-to-peercommunication network and synchronize with each other.

In general, an SBR synthesizes an RF beam to point at locations (e.g.beam areas-of-coverage shown in FIG. 10), for durations, and at timesaccording to a scanning pattern, which may be predetermined or dynamic.A pointing location can be identified by the one or more SBRs as a beamindicator (such as a numeric indicator), a location on the floor of afacility in which the SBRs are located, a set of Cartesian or polarcoordinates, or any other suitable location identifier.

In some embodiments, the scan pattern is a sequence of target locationsand an SBR may synthesize beams to point at the different targetlocations based on a timer, a trigger signal generated by the SBR or acontroller, and/or communications from one or more other SBRs. In someembodiments, the scan pattern is at least one target location and atleast one corresponding target-location time, defined as the time atwhich two different SBRs point to the target location. Thetarget-location time may be absolute (for example, 4:00 pm) or relative(for example, ten milliseconds after a trigger or timer signal orcommunication). An SBR may store the scan pattern in memory, receive ascan pattern from a controller (e.g., controller 2120, another SBR, anetwork device, or some other controlling entity), generate a scanpattern using information received from a controller or other SBRs,generate a scan pattern randomly, generate a scan pattern to optimizetag cooperative powering, or generate a scan pattern based on any othersuitable parameter(s). In some embodiments, an SBR's scan pattern may beoverridden temporarily (or permanently) by a controller or another SBR.

The assignment of roles (inventorying or powering) to different SBRs maydepend on history (i.e. whether an SBR was most recently an inventoryingSBR or a powering SBR), the SBR's location, the number of tags the SBRhas inventoried recently or historically, the number of tags all or asubset of the SBRs have inventoried, or any other number or type ofsuitable parameters. In some embodiments the SBR role assignments may bepreset—for example, a particular SBR may be assigned to inventory for aperiod of time, provide power for another period of time, and thenrepeat. In other embodiments SBR role assignments may be dynamic oradaptive, even during an ongoing communication with a tag. For example,one reader may begin and then interrupt a dialog with a tag. Anotherreader may then continue the interrupted dialog with the tag while theoriginal reader provides power or performs some other task.

In some embodiments, SBRs may be configured to cooperatively power tagsonly in certain circumstances. For example, if one SBR is using one ofits outer beams to inventory tags then other SBR may cooperatively powerthe tags, knowing that cooperative powering is particularly effectivewhen used with outer SBR beams. As another example, if a tag is movingtoward the periphery of an SBR's field-of-view (and therefore into itsouter beam coverage), other SBR may cooperate to provide additionalpower. As yet another example, if the number of inventoried tags is lessthan an expected number, other SBRs may provide additional power toboost tag sensitivity and thereby allow more tags to be inventoried. Asyet another example, if a tag indicates that it has insufficient powerto perform an operation, such as writing data to memory, other SBRs mayprovide additional power so the tag can perform the operation.

As described above, SBRs may be configured to receive and/or exchangeinformation about target locations, scan patterns, scan timing, beamconfiguration, tags, cooperative powering, and roles. FIG. 22 depicts avariety of ways in which SBRs can receive and/or exchange suchinformation. Diagram 2200 depicts a first configuration in which amaster SBR 2202 coordinates the operations of two slave SBRs 2204 and2206. Diagram 2240 depicts a second configuration in which three SBRs,2242, 2244, and 2246 coordinate operation via peer-to-peercommunications. Diagram 2280 depicts a third configuration in which acentralized controller 2282 coordinates the operations of three SBRs2284, 2286, and 2288. Of course, multiple variants on these themes arepossible including using more or less SBRs; mixing the configuration(for example, a controller coordinating peer-to-peer communications);using multiple controllers, and endless other combinations as will beobvious to those of ordinary skill in the art. Communication betweenSBRs and controllers (if present) can be implemented using a wiredconnection (e.g., Ethernet, parallel, serial, or other suitable wiredprotocol), a wireless connection (e.g. WiFi, cellular, Bluetooth, orother suitable wireless protocol), a point-to-point protocol, a packetor address-based protocol, or any other suitable connection type orprotocol.

FIG. 23 depicts a process 2300 for coordinating SBR operation accordingto embodiments. Process 2300 begins at operation 2302, where a first SBR(e.g., SBR 2000) synthesizes a beam (e.g., beam 2002) to point at afirst location. At operation 2304, the first SBR transmits a minimallymodulated (power) signal. At operation 2306, a second SBR (e.g., SBR1900) synthesizes a beam (e.g., beam 1902) to also point at the firstlocation. At operation 2308, the second SBR inventories one or more tagsat the first location while the first SBR continues to transmit power.At optional operation 2310, first and second SBRs repeat the aboveoperations while pointing at a second location, for example as describedabove in FIG. 21. At optional operation 2312, the first and second SBRsmay switch roles, with the first SBR inventorying tags while the secondSBR transmits power. Of course, in other embodiments more than two SBRsmay participate in process 2300.

The SBR functionality described above may be combined with other tagprocesses. For example, cooperative powering may be combined with theresponse-rate-based tag location processes described above. As anotherexample, cooperative powering may be combined with aforward-error-correction tag operating mode in order to increase readersensitivity. In the latter example, tags may be configured or instructed(e.g., via a reader command) to use forward error correction (FEC) whilebackscattering responses. When using FEC, tags may include anerror-correction code (ECC), such as a block code or a convolutionalcode, in backscattered responses. Readers may be able to more easily(e.g., with higher probability of success) recover backscattered tagresponses including FEC than backscattered tag responses without FEC.

In some embodiments, cooperative powering may be combined with tagpopulation management processes to improve inventory performance anddetect tag movement. One such tag population management technique is atag refresh, also described in U.S. Pat. No. 9,330,284, entitled“Broadcast Refresh of RFID Tag Persistence” and issued on May 3, 2016,which is hereby incorporated by reference in its entirety. Tags capableof executing refresh operations can, upon receiving an appropriatereader command or meeting certain criteria, increase the persistencetime of one or more tag flags. Tags may increase flag persistence timeby adjusting a flag physical parameter, such as voltage, current,charge, and/or flux, as described in the above-referenced patent.

The tag refresh functionality may assist in determining the direction ofa tag's movement in an environment that contains multiple other tags.FIG. 24 is a conceptual diagram 2400 showing side views of an SBR systemat different stages of a tag motion tracking process according toembodiments.

In diagram 2400, an SBR 2402 is configured to generate outer RF beams2412, 2414, 2418, and 2420, and a central RF beam 2416. SBR 2402typically scans among its different beams 2412-2420 (and also betweenother beams not depicted in FIG. 24) continuously, to look for tagswithin its coverage area (e.g., as depicted in diagram 1080).

At stage 2410 of the tag-motion tracking process, SBR 2402 firstinventories stationary tags S1-S4 in container 2422 and stationary tagsS5-S6 in container 2424 in a first session. SBR 2402 then broadcasts arefresh signal to the stationary tags to keep them quiet in the firstsession. In some embodiments, these stationary tags may also beinventoried in a second session. In some embodiments the inventoriedflags in the second session do not decay while the tags receive power.

At subsequent stage 2430, a container 2432 with tags of interest (TOIs)2434, 2436, and 2438 moves rightward into SBR 2402's coverage area. Asthey enter the coverage area, SBR 2402 inventories these TOIs with outerRF beams 2412 and 2414, using the second session for the inventorying.Upon observing these new, previously unseen TOIs, SBR 2402 begins atracking process that includes using an alternative session that isdifferent from the second session (such as the first session, althoughany session other than the second may be used).

An inventorying process (such as described in the UHF Gen2Specification) involves a series of steps involving the exchange ofinformation between a reader (such as SBR 2402) and a tag. The processcauses state changes in the reader and/or tag—for example, the readermay request an identifier from the tag, the tag may reply with itsidentifier, the reader may acknowledge receipt of the identifier, andthe tag may then assert an inventoried flag in response to theacknowledgement. In some embodiments a reader may wish to inventory atag without causing the tag to assert its inventoried flag. The readermay accomplish non-acknowledgment by beginning the inventorying processas described above, but either not acknowledge receipt of thetag-provided identifier or transmitting a non-acknowledgement command tothe tag. The NAK command in the UHF Gen2 Specification is one suchnon-acknowledgment command.

At stage 2440, SBR 2402 inventories tags with central beam 2416 usingthe alternative session. Because TOIs 2434-2438 have not yet beeninventoried in the alternative session, their inventoried flags shouldbe in the A state (i.e. denoting not inventoried), and they shouldrespond to SBR 2402. When SBR 2402 receives an identifier from one ofthese TOIs 2434-2438, which it previously determined to be new andof-interest, SBR 2402 transmits a non-acknowledgement command to theTOI, thereby causing the TOI not to change the state of its inventoriedtag and thus facilitating subsequent reinventorying. Moreover, becauseSBR 2402 previously inventoried the TOI with its outer RF beams and isnow inventorying the TOI with its central RF beam, it may infer that theTOI has moved from the coverage area of outer RF beams 2412/2414 to thecoverage area of central RF beam 2416.

As mentioned above, an RFID reader (such as SBR 2402) mayunintentionally miss (i.e. not inventory) a tag (such as one of TOIs2434-2438) in one of its beams. To compensate for such misses. SBR 2402may associate TOIs that it believes to be traveling together, such asTOIs 2434-2438, in a set. SBR 2402 may form this belief from the factthat it inventoried TOIs 2434-2438 at the same time, from tracking TOIs2434-2438 moving together for a period of time, from informationprovided to it about TOIs 2434-2438, from similar TO identifiers, orfrom other information or sources or characteristic of the TOIs. Byassuming the association among the TOIs, even if SBR 2402 misses one ofthe TOIs in a beam, such as in beam 2440, it may still assume that themissed TOI is moving with the others in the set.

At stage 2450, SBR 2402 inventories TOIs 2434-2438 with outer RF beam2420. Similar to stage 2440, SBR 2402 may inventory the TOIs in thealternative session and, after receiving a TOI identifier anddetermining that the TOI was previously inventoried, may transmit anon-acknowledgement command to the TOI.

In the example of FIG. 24, by first inventorying the tag in the secondsession, with all static tags already in the B state, SBR 2402 canquickly identify new TOIs. By using non-acknowledgment commands in thealternative session, SBR 2402 can continue to observe the TOIs in a seaof static tags. By tracking a TOI across beams 2412, 2414, 2416, and2420, SBR 2402 and may infer that a TOI is moving.

Whereas the example in FIG. 24 depicts tracking TOIs moving linearly inone direction (rightward), SBR 2402 may be configured to track TOIsmoving in other directions, in linear or nonlinear paths. For example,SBR 2402 can be configured to track TOIs that enter its coverage areaalong one axis and leave along another (i.e., TOIs that change movementdirection within the SBR coverage area). Similarly, SBR 2402 can beconfigured to track TOIs that move in a curved path.

In some embodiments, an SBR may use tag refresh commands as describedabove to assist in tag tracking. For example, SBR 2402 may use refreshcommands to maintain the inventoried flags of the static tags incontainers 2422 and 2424. FIG. 25 shows a timing diagram 2500 for a tagtracking process with tag refresh commands according to embodiments.Timing diagram 2500 displays the values of the session one (S1) and two(S2) flags a TOI 2502 and two static tags 2504 and 2506. In timingdiagram 2500, a flag is asserted when its value is high and not assertedwhen its value is low. The horizontal axis of the timing diagram 2500represents time, with events to the left preceding events to the right.

At initial time 2510, an SBR inventories static tags 2504 and 2506 inboth S1 and S2 sessions and instructs them to assert their S1 and S2flags. Whereas timing diagram 2500 depicts these tags being inventoriedin both sessions at the same time, in typical embodiments the tags arefirst inventoried in one session and then in the other session.

At subsequent time 2512, the SBR transmits a refresh command to statictags 2504 and 2506 as described above. Without the refresh command attime 2512, the S1 flags of static tags 2504/2506 would decay (asdepicted by the dotted curves) shortly after time 2512. In the depictedembodiment, the S2 flags of the static tags 2504/2506 does not decay,because a tag's S2 flag value persists when the tag is powered (asdescribed in the Gen2 Specification).

At time 2514, the SBR inventories TOI 2502 in session S2 and causes itsS2 session flag to be asserted as a result of being inventoried.

At time 2516, the SBR transmits another refresh command to static tags2504/2506 to maintain their S1 flag values, which would otherwise decay.

At time 2518, the SBR inventories TOI 2502 in session S1. Afterreceiving an identifier from TOI 2502, the SBR transmits anon-acknowledgement command (such as a Gen2 NAK command), causing TOI2502's S1 session flag to remain deasserted.

At times 2520 and 2522 the SBR transmits refresh commands to maintainthe S1 session flag values of static tags 2504 and 2506. At time 2524,the SBR again inventories TOI 2502 in session S1 and terminates theinventorying process by transmitting a NAK command, leaving TOI 2502'sS1 session flag of deasserted.

By first using session S2 to find TOI 2502 among static tags whose S2session flags are held asserted by being powered, and then using sessionS1 and NAKs to read TOI 2502 multiple times among static tags whose S1session flags are refreshed, the SBR is able to inventory TOI 2502, inmultiple beams as described above, and track its movement. Of course,the above session-flag choices are arbitrary—session flags S1 and S2could be swapped, or session flag S3 could be used instead of sessionflag S1, or session flag S3 could be used instead of session flag S2, orthe tags could have customer session flags with different names andattributes.

FIG. 26 is a flowchart of a tag tracking process 2600, as may beperformed by an SBR such as SBR 2402, according to embodiments.

In step 2602, the SBR determines whether to inventory tags or transmit aflag refresh command. The SBR may determine whether to transmit arefresh command based on whether it has recently inventoried any TOIs,the time since the last refresh command, the number of static tags, orany other suitable condition or combination of conditions.

If the SBR chooses to send a refresh command then it transmits a refreshcommand in step 2606, as described above in relation to step 2406 inFIG. 24. If the SBR chooses to inventory tags then it may receive anidentifier from a static tag or a TOI. The SBR may determine whether atag is static or a TOI based on any of the above-described criteria,such as whether the SBR has observed the tag previously, whether the tagis moving, whether the tag has an identifier of interest, etc. The SBRmay use multiple of its RF beams to inventory tags, and may use one ormore sessions. Unless the SBR observes a TOI, it returns to step 2602.

If the SBR finds a potential TOI then, in step 2608, it determines ifthe TOI was previously observed. If confirmed then the SBR may evaluatea TOI parameter (e.g., whether the TOI is moving, which direction itcame from/is going, speed, path, etc.) in step 2610. If the TOI is notconfirmed then the SBR returns to step 2602 without evaluating a TOIparameter. Subsequently, the reader returns to step 2602.

In some embodiments, the SBR prioritizes the order in which it performsthe tasks in step 2602 either in a fixed sequence or dynamically, basedon information received from tags or from external sources.

Whereas the above tag-tracking process uses session S1, which decaysover time, in other embodiments the process may use a different sessionthat does not decay while the tag is energized, such as S3. In theseembodiments refresh step 2606 may be omitted.

The steps described in processes 1800, 2300, and 2600 are forillustration purposes only. RFID tag management using SBRs may beperformed employing additional or fewer steps and in different ordersusing the principles described herein. Of course, the order of the stepsmay be modified, some steps eliminated, or other steps added accordingto other embodiments.

Whereas in the above description the RF beams for transmitting andreceiving are synthesized by an SBR, in some embodiments one or more ofthe beams, in either or both of the transmit and receivefunctionalities, may be generated without the use of a synthesized-beamantenna. For example, the transmit beams may be generated by asynthesized-beam antenna but the receive beam may employ a staticantenna such a a patch, monopole, dipole, etc. As another example, thesynthesized beams may be replaced by multiple static antennas coupled toone or more readers.

According to some examples, a method to estimate a location of a RadioFrequency Identification (RFID) integrated circuit (IC) coupled to anantenna is provided. The method includes generating multipleradio-frequency beams, where each beam is directed to a different targetlocation, transmitting multiple interrogating signals on each beam, andreceiving, on each beam, at least one response from the IC to theinterrogating signals. The method may further include determining aresponse rate for each beam, selecting a first beam having a firstresponse rate and a second beam having a second response rate, where thefirst beam partially overlaps the second beam to form an overlap region,and using a target location of the first beam, a target location of thesecond beam, and the first and second response rates to estimate the IClocation within the overlap region.

According to some embodiments, a ratio of a first distance between thefirst beam target location and the estimated IC location to a seconddistance between the second beam target location and the estimated IClocation may correspond to a ratio of the first and second responserates. The method may further include transmitting the multipleinterrogating signals at multiple radio frequencies and determining thefirst and second response rates at a first one of the multiple radiofrequencies. A ratio of the first distance to the second distance maycorrespond to both a ratio of the first response rate to the secondresponse rate and a ratio of a third response rate to a fourth responserate, where the third response rate is associated with the first beam,the fourth response rate is associated with the second beam, and thethird and fourth response rates are determined at a second radiofrequency different from the first radio frequency. The method mayfurther include selecting the first radio frequency based on anenvironmental condition, a previously received IC response, aperformance characteristic of the IC, and/or a tag population size. Themethod may further include refining the estimated IC location bycomparing a sensitivity contour of the first and/or second beams to afrequency-dependent operating profile of the IC and/or a tag includingthe IC.

According to other embodiments, the method may further include selectinga third beam having a third target location and a third response rate,where the third beam partially overlaps the first and second beams toform the overlap region. The IC location estimation may be done usingthe first, second, and third beam target locations and the first,second, and third response rates. A ratio of the first distance to thesecond distance may correspond to a ratio of the first and secondresponse rates, and a ratio of the first distance to a third distancebetween the third beam target location and the estimated IC location maycorrespond to a ratio of the first and third response rates. The methodmay further include generating the multiple beams from asynthesized-beam RFID reader and/or multiple RFID readers. Each beamresponse rate may be a count of responses received on the respectivebeam divided by the number of interrogating signals transmitted on therespective beam. The method may further include determining excludedlocations for the RFID IC based on map information and removing theexcluded locations from the region.

According to other examples, a method to estimate a location of an RFIDIC coupled to an antenna is provided. The method includes generatingmultiple pairs of radio-frequency (RF) beams, where each beam pair isdirected to a different target location and the beams within each beampair cooperatively provide RF power to the target location, transmittingmultiple interrogating signals on at least one beam of each beam pair,and receiving, on at least one beam of each beam pair, at least oneresponse from the IC to the interrogating signals. The method mayfurther include determining a response rate for each beam pair,selecting a first beam pair having a first response rate and a secondbeam pair having a second response rate, where the first beam pairpartially overlaps the second beam pair to form an overlap region, andusing a target location of the first beam pair, a target location of thesecond beam pair, and the first and second response rates to estimatethe IC location within the overlap region.

According to some embodiments, one beam of at least one pair maytransmit the interrogating signal and the other beam may transmit asubstantially unmodulated continuous-wave signal, and/or both beams ofat least one pair may transmit synchronized interrogating signals. Aratio of a first distance between the first beam pair target locationand the estimated IC location to a second distance between the secondbeam pair target location and the estimated IC location may correspondto a ratio of the first and second response rates. The method mayfurther include transmitting the multiple interrogating signals atmultiple radio frequencies and determining the first and second responserates at a first one of the multiple radio frequencies. In someembodiments, a ratio of the first distance to the second distance maycorrespond to both a ratio of the first response rate to the secondresponse rate and a ratio of a third response rate to a fourth responserate, where the third response rate is associated with the first beampair, the fourth response rate is associated with the second beam pair,and the third and fourth response rates are determined at a second radiofrequency different from the first radio frequency. The method mayfurther include selecting the first frequency based on an environmentalcondition, previously received IC replies, a performance characteristicof the RFID IC, and/or a tag population size. The method may furtherinclude selecting transmitting radio frequencies for each beam in a pairto avoid generating beat notes. The method may further include selectinga third beam pair having a third target location and a third responserate, where the third beam pair partially overlaps the first and secondbeam pairs to form the overlap region. The IC location estimation may bedone using the first, second, and third beam target locations and thefirst, second, and third response rates. A ratio of the first distanceto the second distance may correspond to a ratio of the first and secondresponse rates, and a ratio of the first distance to a third distancebetween the third target location and the estimated IC location maycorrespond to a ratio of the first and third response rates.

According to further examples, a method for an RFID synthesized-beamreader to estimate a location of an RFID IC coupled to an antenna isprovided. The method includes serially synthesizing each of multiplebeams according to a scan pattern, where each beam is directed to adifferent target location, transmitting a series of interrogatingsignals on each beam, and receiving, on each beam, at least one responsefrom the IC to the series of interrogating signals. The method mayfurther include determining a response rate for each beam, selecting afirst beam having a first response rate and a second beam having asecond response rate, where the first beam partially overlaps the secondbeam to form an overlap region, and using a target location of the firstbeam, a target location of the second beam, and the first and secondresponse rates to estimate the IC location within the overlap region.

According to some embodiments, the method may further include, for eachbeam, discriminating, based on the beam's response rate, whether the ICis in a sidelobe of the beam, and if the IC is in the sidelobe, thenexcluding the beam's response rate when estimating the location of theIC.

The foregoing detailed description has set forth various embodiments ofthe devices and/or processes via the use of block diagrams and/orexamples. Insofar as such block diagrams and/or examples contain one ormore functions and/or aspects, it will be understood by those within theart that each function and/or aspect within such block diagrams orexamples may be implemented individually and/or collectively, by a widerange of hardware, software, firmware, or virtually any combinationthereof. Those skilled in the art will recognize that some aspects ofthe embodiments disclosed herein, in whole or in part, may beequivalently implemented employing integrated circuits, as one or morecomputer programs running on one or more computers (e.g., as one or moreprograms running on one or more computer systems), as one or moreprograms running on one or more processors (e.g. as one or more programsrunning on one or more microprocessors), as firmware, or as virtuallyany combination thereof, and that designing the circuitry and/or writingthe code for the software and or firmware would be well within the skillof one of skill in the art in light of this disclosure.

The present disclosure is not to be limited in terms of the particularembodiments described in this application, which are intended asillustrations of various aspects. Many modifications and variations canbe made without departing from its spirit and scope, as will be apparentto those skilled in the art. Functionally equivalent methods andapparatuses within the scope of the disclosure, in addition to thoseenumerated herein, will be apparent to those skilled in the art from theforegoing descriptions. Such modifications and variations are intendedto fall within the scope of the appended claims. The present disclosureis to be limited only by the terms of the appended claims, along withthe full scope of equivalents to which such claims are entitled. It isto be understood that this disclosure is not limited to particularmethods, configurations, antennas, transmission lines, and the like,which can, of course, vary. It is also to be understood that theterminology used herein is for the purpose of describing particularembodiments only, and is not intended to be limiting.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

It will be understood by those within the art that, in general, termsused herein, and especially in the appended claims (e.g., bodies of theappended claims) are generally intended as “open” terms (e.g., the term“including” should be interpreted as “including but not limited to,” theterm “having” should be interpreted as “having at least,” the term“includes” should be interpreted as “includes but is not limited to,”etc.). It will be further understood by those within the art that if aspecific number of an introduced claim recitation is intended, such anintent will be explicitly recited in the claim, and in the absence ofsuch recitation no such intent is present. For example, as an aid tounderstanding, the following appended claims may contain usage of theintroductory phrases “at least one” and “one or more” to introduce claimrecitations. However, the use of such phrases should not be construed toimply that the introduction of a claim recitation by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim recitation to embodiments containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g., “a” and/or “an” should be interpreted to mean “at least one”or “one or more”); the same holds true for the use of definite articlesused to introduce claim recitations. In addition, even if a specificnumber of an introduced claim recitation is explicitly recited, thoseskilled in the art will recognize that such recitation should beinterpreted to mean at least the recited number (e.g., the barerecitation of “two recitations,” without other modifiers, means at leasttwo recitations, or two or more recitations).

Furthermore, in those instances where a convention analogous to “atleast one of A, B, and C, etc.” is used, in general such a constructionis intended in the sense one having skill in the art would understandthe convention (e.g., “a system having at least one of A, B, and C”would include but not be limited to systems that have A alone, B alone,C alone, A and B together, A and C together, B and C together, and/or A,B, and C together, etc.). It will be further understood by those withinthe art that virtually any disjunctive word and/or phrase presenting twoor more alternative terms, whether in the description, claims, ordrawings, should be understood to contemplate the possibilities ofincluding one of the terms, either of the terms, or both terms. Forexample, the phrase “A or B” will be understood to include thepossibilities of “A” or “B” or “A and B.”

As will be understood by one skilled in the art, for any and allpurposes, such as in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” “greater than,” “less than,” and the likeinclude the number recited and refer to ranges which can be subsequentlybroken down into subranges as discussed above. Finally, as will beunderstood by one skilled in the art, a range includes each individualmember. Thus, for example, a group having 1-3 cells refers to groupshaving 1, 2, or 3 cells. Similarly, a group having 1-5 cells refers togroups having 1, 2, 3, 4, or 5 cells, and so forth.

We claim:
 1. A method to estimate a location of a Radio FrequencyIdentification (RFID) integrated circuit (IC) coupled to an antenna, themethod comprising: generating multiple radio-frequency beams, eachdirected to a different target location; transmitting multipleinterrogating signals on each beam; receiving, on each beam, at leastone response from the IC to the interrogating signals; determining, foreach beam, a response rate; selecting a first beam having a firstresponse rate and a second beam having a second response rate, whereinthe first beam partially overlaps the second beam to form an overlapregion; and using a target location of the first beam, a target locationof the second beam, and the first and second response rates to estimatethe IC location within the overlap region.
 2. The method of claim 1,wherein a ratio of a first distance between the first beam targetlocation and the estimated IC location to a second distance between thesecond beam target location and the estimated IC location corresponds toa ratio of the first and second response rates.
 3. The method of claim1, further comprising: transmitting the multiple interrogating signalsat multiple radio frequencies; and determining the first and secondresponse rates at a first one of the multiple radio frequencies.
 4. Themethod of claim 3, wherein a ratio of a first distance between the rustbeam target location and the estimated IC location and to a seconddistance between the second beam target location and the estimated IClocation corresponds to both: a ratio of the first response rate to thesecond response rate; and a ratio of a third response rate to a fourthresponse rate, wherein the third response rate is associated with thefirst beam, the fourth response rate is associated with the second beam,and the third and fourth response rates are determined at a second radiofrequency different from the first radio frequency.
 5. The method ofclaim 3, further comprising selecting the first radio frequency based onat least one of an environmental condition, a previously received ICresponse, a performance characteristic of the IC, and a tag populationsize.
 6. The method of claim 1, further comprising refining theestimated IC location by comparing a sensitivity contour of at least oneof the first and second beams to a frequency-dependent operating profileof at least one of the IC and a tag including the IC.
 7. The method ofclaim 1, further comprising: selecting a third beam having a thirdtarget location and a third response rate, wherein the third beampartially overlaps the first and second beams to form the overlapregion, and using the first, second, and third beam target locations andthe first, second, and third response rates to estimate the IC location,wherein: a ratio of a first distance between the first beam targetlocation and the estimated IC location to a second distance between thesecond beam target location and the estimated IC location corresponds toa ratio of the first and second response rates; and a ratio of the firstdistance to a third distance between the third beam target location andthe estimated IC location corresponds to a ratio of the first and thirdresponse rates.
 8. The method of claim 1, further comprising generatingthe multiple beams from at least one of a synthesized-beam RFID readerand multiple RFID readers.
 9. The method of claim 1, wherein each beamresponse rate is a count of responses received on the respective beamdivided by the number of interrogating signals transmitted on therespective beam.
 10. The method of claim 1, further comprising:determining, based on map information, excluded locations for the RFIDIC; and removing the excluded locations from the region.
 11. A method toestimate a location of a Radio Frequency Identification (RFID)integrated circuit (IC) coupled to an antenna, the method comprising:generating multiple pairs of radio-frequency (RF) beams, wherein eachbeam pair is directed to a different target location and the beamswithin each beam pair cooperatively provide RF power to the targetlocation; transmitting multiple interrogating signals on at least onebeam of each beam pair; receiving, on at least one beam of each beampair, at least one response from the IC to the interrogating signals;determining, for each beam pair, a response rate; selecting a first beampair having a first response rate and a second beam pair having a secondresponse rate, wherein the first beam pair partially overlaps the secondbeam pair to form an overlap region; and using a target location of thefirst beam pair, a target location of the second beam pair, and thefirst and second response rates to estimate the IC location within theoverlap region.
 12. The method of claim 11, wherein at least one of: onebeam of at least one pair transmits the interrogating signal and theother beam transmits a substantially unmodulated continuous-wave signal;and both beams of at least one pair transmit synchronized interrogatingsignals.
 13. The method of claim 11, wherein a ratio of a first distancebetween the first beam pair target location and the estimated IClocation to a second distance between the second beam pair targetlocation and the estimated IC location corresponds to a ratio of thefirst and second response rates.
 14. The method of claim 13, furthercomprising: transmitting the multiple interrogating signals at multipleradio frequencies, and determining the first and second response ratesat a first one of the multiple radio frequencies.
 15. The method ofclaim 14, wherein estimating the IC location within the overlap regionincludes estimating the IC location such that a ratio of a firstdistance between the IC location and the first beam target location to asecond distance between the IC location and the second beam targetlocation corresponds to both: a ratio of the first response rate to thesecond response rate; and a ratio of a third response rate to a fourthresponse rate, wherein the third response rate is associated with thefirst beam pair, the fourth response rate is associated with the secondbeam pair, and the third and fourth response rates are determined at asecond radio frequency different from the first radio frequency.
 16. Themethod of claim 14, further comprising selecting the first frequencybased on at least one of an environmental condition, previously receivedIC replies, a performance characteristic of the RFID IC, and a tagpopulation size.
 17. The method of claim 14, further comprisingselecting transmitting radio frequencies for each beam in a pair toavoid generating beat notes.
 18. The method of claim 11, furthercomprising: selecting a third beam pair having a third target locationand a third response rate, wherein the third beam pair partiallyoverlaps the first and second beam pairs to form the overlap region, andusing the first, second, and third beam target locations and the first,second, and third response rates to estimate the IC location, wherein: aratio of a first distance between the first beam pair target locationand the estimated IC location to a second distance between the secondbeam pair target location and the estimated IC location corresponds to aratio of the first and second response rates; and a ratio of the firstdistance to a third distance between the third target location and theestimated IC location corresponds to a ratio of the first and thirdresponse rates.
 19. A method for a Radio Frequency Identification (RFID)synthesized-beam reader (SBR) to estimate a location of an RFIDintegrated circuit (IC) coupled to an antenna, the method comprising:serially synthesizing each of multiple beams according to a scanpattern, each beam directed to a respective target location;transmitting a series of interrogating signals on each beam; receiving,on each beam, at least one response from the RFID IC to the series ofinterrogating signals; determining, for each beam, a response rate;selecting a first beam having a first response rate and a second beamhaving a second response rate, wherein the first beam partially overlapsthe second beam to form an overlap region; and using a target locationof the first beam, a target location of the second beam, and the firstand second response rates to estimate the IC location within the overlapregion.
 20. The method of claim 19, further comprising, for each beam:discriminating, based on the beam's response rate, whether the IC is ina sidelobe of the beam; and if the IC is in the sidelobe, then excludingthe beam's response rate when estimating the location of the IC.